The present invention relates to a frequency-conversion mixer and more particularly to a frequency-conversion mixer which operates within substantially the whole range of the radio frequency band extending from long wave or low-frequency to ultra-high-frequency.
The frequency-conversion mixers heretofore used in superheterodyne receivers, transverter transmitters, etc., have included vacuum-tube mixers, transistor mixers, diode mixers, double balanced mixers, etc.
The vacuum-tube mixers have consisted of a triode, pentode or heptode and have had the disadvantages of causing conversion noise inherent to the mixer, producing a greater loss than the diode mixers at frequencies higher than an upper-limit frequency of several hundreds MHz due to the construction and size of the electrodes of the vacuum tube and requiring a very critical coupling between the mixer and the local oscillator. In any event, vacuum-tube mixers have come out of use along with the development of transistors.
Transistor mixers have substantially the same electric performance as vacuum-tube mixers but even somewhat greater conversion noise. In addition, while mixers employing field-effect transistors have been improved more or less in cross modulation distortion characteristic as compared with other transistor mixers, they still have substantially the same conversion loss as vacuum-tube mixers.
Referring now to FIG. 7, there is shown a type of the diode mixers which has been used in ordinary transmitters, radars, etc., involving frequencies higher than the ultra-high frequency band. A signal R of a frequency f.sub.1, which has been received through a tuning circuit 1, is coupled with a local-oscillator signal OSC of a frequency, mixed by a diode 2 and outputted as an intermediate frequency signal IF of a frequency f.sub.1 +f.sub.2 or .vertline.f.sub.1 -f.sub.2 .vertline. through a tuning circuit 3. Such diode mixers have large conversion loss and, frequently, coupling loss due to the inevitable use of magnetic coupling from the tuning circuit and thus restrictions are imposed on obtaining the best coupling.
Also, FIG. 8 shows a circuit diagram of the double balanced mixer. The double balanced mixer includes 4 diodes D.sub.1, D.sub.2, D.sub.3 and D.sub.4 which are connected in a ring form so that, when a received signal R is applied via an input-output transformer T.sub.1 and also a local-oscillator signal OSC is applied between the secondary side of the input-output transformer T.sub.1 and the primary side of an input-output transformer T.sub.2, an intermediate frequency signal IF of a frequency f.sub.1 +f.sub.2 or .vertline.f.sub.1 -f.sub.2 .vertline. is generated through the input-output transformer T.sub.2.
The double balanced mixer has been designed so that the local-oscillator signal OSC does not leak from the mixer circuit. As a result, the cross modulation distortion characteristic is excellent as compared with the vacuum mixer, the transistor mixer and the diode mixer. In addition, the coupled circuit can be coupled directly at X. Presently, therefore, it is the best mixer.
As another result, if the circuit design of a double balanced mixer is proper, the mixer circuit itself operates positively and no adjustment is required. Thus, the double balanced mixer is used in a variety of circuits.
However, double balanced mixers have the disadvantage that the electric characteristics are largely dependent on the characteristics of the component parts such as the diodes and the transformers thus requiring the careful selection of component parts depending on the frequency band used.
Also, it has been required that the output of the local-oscillator signal is between 10 and 30 dbm. There has been another disadvantage that there are instances where the output of the local-oscillator signal corresponds to the transmitted output in microwave and therefore a high cost is required just by the circuit for generating the local-oscillator signal.
Further, the double balanced mixers have been disadvantageous in that they are highest in conversion loss as compared with the mixers of the other types thus requiring the provision of an amplifier on the output side.